1. Field of the Invention
This invention is in the field of treating arsenic-bearing materials such as tin or other metal ores which involves reacting the material to form a specific type of arsenic sulfide controlled conditions and then condensing the sulfide so produced to remove it from the system without polluting the surrounding environment.
2. Description of the Prior Art
In the processing of arsenic-containing ores, it is necessary for metallurgical reasons to separate the arsenic from the metals or other intermediate products to be obtained. After initial separation of the arsenic, usually in the gas phase in the form of arsenic oxide, an additional step is necessary in order to either obtain the arsenic in the form of a metal arsenide or, for reasons of keeping the environment clean, to convert it into a disposable, harmless or non-toxic end product.
Due to increased demands regarding the environment, measures must be taken on existing metallurgical systems and those which are to be newly constructed, to guarantee a complete and as selective as possible a separation of the arsenic and other toxic materials. These measures are particularly necessary in view of the extension of raw material sources into ores or other raw materials which because of their high arsenic concentrations present technical difficulties.
In the case of removing arsenic during calcining and reduction of heavy metal ores, the arsenic is volatilized into a gas phase predominantly in the oxide form as As.sub.4 O.sub.6, and is then either partially condensed as As.sub.2 O.sub.3 or dissolved in an aqueous phase. This is followed by the precipitation of the arsenic in the form of an arsenate, most frequently in the form of calcium arsenate or iron arsenate. The condensation of arsenic-containing vapors leads to the formation of liquid layers which may lead to clogging in the exhaust gas conduction system. The removal of arsenic-oxide-deposits from filter systems and exhaust gas pipes is dangerous to the health of operating personnel and is very costly. The calcium arsenates which are formed by treating the arsenic-containing washing liquid with milk of lime exhibit solubilities in water of about 0.1 to 0.3% which is far above the maximum required for keeping the environment clean.
In German AS No. 19 34 440 there is described a method for the removal of arsenic and non-ferrous metals from calcined pyrites which involves a partial reduction of the roasted material by means of a carbonaceous fuel and air, a subsequent chlorination of the thus treated hot ores in a fluid bed reactor with a gas mixture consisting of chlorine, oxygen, and an inert gas for 30 to 120 minutes at a temperature of 650.degree. to 950.degree. C. The gas employed for the purpose of chlorination contains an oxygen concentration of greater than 3% by volume. After the chlorination, the soluble arsenate remains in the solid particles. The gas phase exhibits volatilized trivalent arsenic oxide and trivalent arsenic chloride which are then removed by a wash water which nevertheless contains a high percentage of the arsenic compounds.